Sorption of Arsenic by Iron Sulfide Made by Sulfate-reducing Bacteria: Implications for Bioremediation
Abstract
In this study, data from field bioremediation experiments, geochemical modeling, and laboratory batch experiments were integrated with published data on arsenic (As) sorption and coprecipitation onto growing iron (Fe)-sulfide phases [e.g., iron monosulfide, FeS, pyrite, FeS2, and arsenian pyrite, Fe(S, As)2] to characterize geochemical processes during in situ bioremediation of natural As-contaminated groundwater. Field bioremediation experiments conducted in the past on groundwater in Holocene alluvial aquifers in Bangladesh and the United States (US) have shown that As was incorporated into Fe-sulfide phases in reducing groundwater, and that this process can be fast as well as efficient. This study shows that As can be removed in a matter of weeks after the injection of water-soluble labile organic carbon and sulfate that stimulate metabolism of indigenous sulfate- reducing bacteria (SRB) in Fe-bearing, low-temperature, reduced, As-contaminated groundwater. A new set of thermodynamic data for thiroarsenite species, amorphous arsenic and Fe-sulfide phases, and solid solution of arsenian pyrite (FeS1.99As0.01 – FeS1.90As0.10) were complied into a revised Geochemist’s Workbench (GWB) database, Thermo08-As, to model the principal geochemical behavior of As in aerobic and anaerobic groundwaters. Compared to the most widely used geochemical modeling programs, which lack thermodynamic data for solid solutions of arsenian pyrite, this new thermodynamic database is more realistic in characterizing and predicting As behavior in changing redox conditions. Under Fe-rich geochemical conditions, the stability field of arsenian pyrite (containing 1 to 10 wt.% As) solid solution completely dominates in reducing Eh-pH space and “displaces” other As-sulfides (orpiment, realgar) that have been implied to be important in previous modeling and field studies. Sorption of dissolved As in synthetic Fe-sulfide and natural pyrite as a function of total As concentration, sulfide ratio, Eh-pH, time, and grain size of pyrite, were investigated in the laboratory. Arsenic is strongly partitioned on both FeS and FeS2 under a range of conditions, such as pH and As concentration. In the sulfide-limited (S:Fe=1:1) experiment that produced synthetic FeS, 91% of the initial dissolved As, was sorbed. In contrast, in the excess-sulfide (S:Fe=2:1 and 3:1) experiment, 55% of the initial As concentration was sorbed, but yielded pyrite as a solid phase. Amount of As sorbed onto pyrite is dependent on grain size, but conformed to a Langmuir isotherm at circumneutral pH. Field data, geochemical modeling, and laboratory results clearly indicate that As is mobile under Fe-reducing conditions, but immobile under anaerobic and sulfate-reducing conditions. Fe-oxyhydroxides and arsenian pyrite are the likely stable mineral phases that serve as major sink for As under aerobic and sulfate-reducing conditions, respectively.